# How long can I power my device using a battery?

Lets say I have a battery that has 100mAh, and I am using it to power an IC that according to it's datasheet, consumes 1mA.

I understand that if I were to connect my IC to a bench power supply I would see it consumes 1mA (if it has a display), and theoretically I could leave it running constantly for months, but what about connecting it to a battery? How can I calculate the minutes/hours I can leave the device running?

Well, 100mAH/1mA = 100 hours. Depending on the battery, its output may droop to an unusable level before the 100 hours. NiMH or Li based batteries have the output level better before dropping off. 100 hours/24 hrs/day = just over 4 days.

• What I don't understand is, that current consumption of 1mA that is specified on the datasheet - is it per minute? per hour? – nettek Jan 29 '19 at 19:02
• It isn't "per" anything -- it's continuous. Current is defined as the amount of charge (coulombs) that pass in a given amount of time: 1A = 1C per second. That's why battery capacity (charge) is given as current * time. The units of mA and hours are the most convenient, but it could just as well been specified as 360 Coulombs. – Dave Tweed Jan 29 '19 at 19:05
• @Eran Current is a rate quantity. One ampere is one coulomb per second, if you must have a per in there, so 1mA is 1mC/s. – Hearth Jan 29 '19 at 19:10
• If I have a battery of 100mAh, and a load that according to it's datasheet, draws 100mA, does it mean that after connecting the load, the battery will be able to supply 100mA continuously for exactly 1 hour (theoretically)? – nettek Jan 29 '19 at 19:21
• With an ideal battery, sure. With a real battery, doubtful. The 100mA will likely cause the battery output voltage to droop pretty quickly, and once the voltage drops it is not clear how much current the load will continue to draw, or if it will work correctly. For example, an Atmega328P clocked at 16 MHz needs a 3.8V to 5.5V to work correctly. Once 3.8V is reached, it may continue to work correctly for whatever the code is doing, or some parts may appear to have errors (such as serial interface). An example is this: CR2032 battery, 235mAH - but only designed for 0.2mA output current. – CrossRoads Jan 29 '19 at 19:30